Automotive vehicles include brakes associated with the wheels for controlling movement of the vehicle. For many years drum brakes have been used. Disc brakes are also well known.
A typical configuration of a drum brake device includes two brake shoes that are forced outwardly against an inner surface of a drum. While drum brakes typically function reasonably well, they present challenges during assembly and service because of the relatively large number of springs, clips, linkages and adjusters that are used to hold the brake shoes in position.
Three known types of drum brakes include the duo-servo brake, the two leading shoe brake (also known as the duplex brake) and the leading/trailing brake (also known as the simplex brake). The principle differences among these three drum brake types are the way in which the brake torque reaction is handled by the brake shoes. An anchor pin or abutment located at the leading edge of the shoe takes the torque reaction in the individual leading shoe arrangement. The trailing shoe arrangement, by contrast, has an anchor pin at the trailing end of the shoe. In a duo-servo brake, both shoes act in series on a single abutment at the leading end of one of the shoes.
In all of the drum brake geometries described above, some level of self-excitation occurs. The brake application force may be amplified by the generation of tangential braking forces on the surface of the brake lining. While self-excitation was useful before power assisted brake systems were introduced, there may be disadvantages to a self-excited brake geometry. Greater sensitivity to changes in the coefficient of friction of the brake lining may be a disadvantage. Duo-servo brakes, in particular, have been known to experience brake fade associated with such changes. Also, a self-excited brake geometry tends to aggravate brake noise such as squeal.
By contrast, disc brakes typically are configured to apply forces perpendicular to the braking surface and the reaction forces are parallel to the braking surface. Disc brakes typically do not experience an amplification effect and have less sensitivity to variations in the coefficient of friction. Disc brakes, however, generally require some form of power assist. While disc brake arrangements usually avoid the fade resistance characteristic of some drum brakes, disk brakes tend to be more expensive.